Chemistry and Related Properties of Freshly Emitted Sea Spray Aerosol

Clouds, which cover about 60% of the Earth at any given time, have a large impact on the planet’s radiation balance by reflecting solar radiation back to space and trapping infrared radiation emitted by the surface. Clouds only form through the condensation of water vapor onto atmospheric particulates known as cloud condensation nuclei (CCN). Whether a particle is able to act as a CCN depends on its size and chemical composition. Optically thin clouds that persist in marine regions are particularly susceptible to changes in the concentration, size, and composition of atmospheric particulates. The response of thin marine clouds to changes in particulates can be seen in satellite images that reveal brighter cloud regions due to ship emissions. The enhanced particle concentration in ship emissions leads to an increase in the number of smaller cloud drops and an increase in the reflectivity, or albedo, of the cloud. Atmospheric particulates can also impact cloud lifetime, extent, and precipitation.

Sources of CCN to the marine atmosphere include the wind-driven production of sea spray aerosol (SSA). Sea spray aerosol consists of inorganic sea salt and organic matter that is scavenged from surface seawater during the production process. An article recently published in Chemical Reviews, by PMEL scientists Trish Quinn and Tim Bates and co-authors, summarizes the results of recent field and laboratory experiments that have characterized the properties of freshly emitted SSA, with an emphasis on the organic fraction. The fraction of SSA important for cloud formation appears to be the large pool of dissolved organic carbon in surface seawater that results from the decomposition of ocean phytoplankton. Many questions remain about links between seawater properties and the composition and cloud-forming abilities of SSA. A combination of observations and modeling efforts by both the oceanic and atmospheric research communities is needed to inform and improve climate model simulations of the impact of SSA on marine clouds.